This invention relates to a fluid dispensing system for use in automated analytical instruments and, more particularly, to such a system wherein an optical control system is utilized to precisely position the orifice of a pipette tip at a desired dispense position relative to an assay device.
Various types of chemical tests can be performed by automated test equipment, an example of testing of considerable interest being the assay of biological substances for human health care. Automated test equipment allows large numbers of test samples to be processed rapidly. Such equipment is employed in health care institutions including hospitals and laboratories. Biological fluids, such as whole blood, plasma or serum are tested to find evidence of disease, to monitor therapeutic drug levels, etc.
In the automated test instrument a sample of the test fluid is typically provided in a sample cup and all of the process steps including pipetting of the sample onto an assay test element, incubation and readout of the signal obtained are carried out automatically. All the process steps can be carried out while the assay test element is carried by a conveyor within a temperature controlled chamber. Further, in such instruments disposable pipette tips are typically used for the delivery of one fluid only and then discarded so as to avoid contamination which could lead to errors in the assay result. It is necessary, when dispensing sample fluid and/or test reagent(s) to the assay element that the orifice of the pipette tip be located at a predetermined, precisely controlled location above the assay element.
This requirement can be better understood from the following discussion of the typical manner in which a fluid dispensing system operates in a typical automated analytical instrument. The fluid dispensing system which includes a pipette is used to transfer sample fluid and/or test reagents from storage cups or wells within the instrument to the assay test element. The pipette includes a hollow tube or stem typically of metal, which is adapted to cooperate with a disposable pipette tip, which is typically made of a polymeric material. The disposable pipette tips are provided in a storage tray within the instrument. Initially, the pipette is advanced downwardly to secure a disposable tip by frictional contact. Thereafter, a predetermined amount of fluid is aspirated into the pipette tip and the tip is then moved automatically to a dispense position above an assay test element where a predetermined volume of the fluid is dispensed to the assay element. Upon completion of the dispense step the tip is discarded and a clean disposable tip is used for the next dispense step.
A problem can arise in the use of such a fluid dispense system due to the fact that each disposable tip is positioned on the metal stem of the pipette by a frictional fit. Since the polymeric materials from which disposable tips are typically made are flexible, there may be some variation from tip to tip as to the distance of the tip orifice from the metal stem of the pipette. Since, as mentioned previously, it is necessary to locate the pipette tip orifice at a predetermined, precisely controlled position above the assay element during the dispense steps, any variation in the positioning of the disposable tip on the pipette stem can result in an error in the desired positioning of the pipette tip which can lead to an error in the assay result.
Accordingly, it would be desirable to provide, in an analytical instrument which utilizes disposable tips in conjunction with a pipette for delivering fluids to an assay test element, the capability of determining the spatial location of the pipette tips during the dispense cycles so as to be able to arrange the orifice of the tips at a predetermined, precisely controlled position irrespective of the distance of the pipette tip orifice from the pipette stem. In addition, it would be advantageous to be able to determine whether the pipette has a disposable tip positioned thereon.